This project has three broad objectives: to define the biochemical and genetic mechanisms which regulate transmembrane transport of amino acids in mammalian cells; to understand how such transport mechanisms control and are controlled by intracellular metabolic pathways for amino acids and their catabolic products; and to translate these observations into diagnostic and therapeutic programs for selected inherited and acquired diseases. Toward these objectives, we will consider four specific problems which follow the flow of amino acids and their catabolites from the cell membrane to the mitochondrion; and which focus on selected enzymes and coenzymes as well as on the amino acids and organic acids whose interconversions they catalyze. These four specific aims will examine: the relationship between the gamma-glutamyl cycle and amino acid transport; the biochemistry and genetics of human ornithine transcarbamylase; the mitochondrial transport and metabolism of vitamin B12; and certain diagnostic and pathophysiologic considerations in the organic acidemias. In some phases of this investigation we will pose or test hypotheses in animals or animal tissues. In others, we will take our clues from sick humans, hoping to follow such inquiry both to new information concerned with the patient's betterment and to a clearer understanding of normal physiology.